Is it totally missing or do you see a column of icons where the menu would be? If you see the icons, the menu is collapsed and can be displayed by clicking the bottom icon — which looks like an arrowhead in a circle.

Only a re-install of WP is turning the Sidebar panel back… and actually it’s not on all my WP 3.8 Websites, but on 5 of them on 10… which is a lot ! and all those
WP missing panel are not even on the same host…

btwimages & AzMaz, Can I ask you each to start a new topic & post as much info as possible as the forums work better when when each topic sticks issue. Even when people have sites with an issue exhibiting similar symptoms, that does not mean that the root cause would necessarily be the same.